1. Field of the Invention
The present invention relates to an automatic focusing device for the use of picture photographing apparatus and others. More particularly, the invention relates to an automatic focusing device capable defining arbitrarily a lens shifting range when an automatic focusing is performed in a picture photographing apparatus and the like.
2. Related Background Art
Traditionally, as an automatic focusing device for the use of a picture photographing apparatus and the like, there has been a device using a system in which a range finding optical system is provided independent of a photographing optical system to detect a position where the contrast is the greatest by scanning from the infinite to the closest or a system in which a focus point is found by a trigonometrical surveying or the like thereby to shift the focusing optical system to such a position found.
In the former case, before the photographing optical system is driven, the entire range is scanned in advance. Therefore, when there is a limit defined to set the range of an object distance for the automatic focusing operation, it is comparatively easy to exclude any objects outside the defined range when the automatic focusing is attempted, (disclosed in Japanese Patent Publication No. 61-20845).
In the latter case, while a rapid automatic focusing is possible because the scanning of the entire range is not needed in measuring the distance, it is usually required to control the shift of the focusing optical system mechanically or the driving of a motor for the automatic focus drive using electrical control signals when a range of an object distance is limited for the automatic focusing operation, (disclosed in Japanese Patent Application Laid-Open No. 58-85406, Japanese Patent Application Laid-Open No. 58-85407, and others).
Meanwhile, in recent years, there have been proposed many automatic focusing devices using the so-called TTL phase difference detecting method in which the displacement of the images formed in the direction perpendicular to the optical axis by two light beams obtainable by dividing a beam transmitted through a photographing optical system is detected, and the defocus amount in the optical axis direction and the defocus direction are obtained in order to shift the focusing optical system for the performance of the automatic focusing.
In an automatic focusing device of the above-mentioned TTL phase difference detecting method, if the current position of the focusing optical system and the position where an object should be focused on an anticipated image formation plane are greatly apart to bring about a large defocus amount, the image contrast becomes low, thus making it impossible to detect the defocus amount. In such a case, the so-called scanning operation is needed so as to find a position where the contrast can be detected by driving the scan between the infinite and the closest in the focusing optical system.
In order to shorten a time required for such a scanning operation for a quicker focusing, a range limit is defined for the object distances for the automatic focusing operation so that no scanning operation is performed for a distance range where the possibility of the object presence is low, such as a region in the closest side having a high photographing magnification. For this purpose, the aforesaid method of controlling the shift of the focusing optical system mechanically or by the driving control of the motor for the automatic focusing drive using electrical control signals should be adequately usable.
However, there is another objective in defining the range limit for the object distances for the automatic focusing operation. Such a range limit is required in a case where another object is present on the same optical axis of the object of a specific target but at a different distance. In such a case, an object to be focused cannot be defined in an usual automatic focusing operation, hence making it impossible to focus on the object of the specific target. Therefore, the range limit of the object distances is set to define the object to be focused within such limit thereby to focus on the target object automatically. For example, there is a person over a wire netting, and when only the person should be focalized for photographing but not the wire netting, the distance farther from the position of the wire netting is set as a limit range. Then, the automatic focusing is reliably performed on the objective person but not on the wire netting, (disclosed in Japanese Patent Application Laid-Open No. 2-118610).
For this purpose, however, the conventional method of controlling the shift of the focusing optical system only is not sufficient. In other words, even when the limit range is set for a distance farther from the wire netting, there is a problem encountered in that the objective person cannot be focalized if an attempt is made to obtain the defocus amount with respect to the wire netting by securing the contrast of the image of the wire netting when the lens is positioned near the closest side limiting position in the limit range. At this position, the system tends to focus on the wire netting and the lens drive is performed towards the closest side.